Both before and since the advent of high temperature superconductors, many investgators have analyzed and experimented to determine the operating characteristics of circuit components at low temperatures. In particular, the operation of MOSFET's, superconducting inductors, and cryocooled capacitors have been examined and thermal models have been developed for these components. For example, power MOSFET's have been operated at temperatures of 77K, and, significant advantages have been noted, such as a reduction of the on-resistance of the MOSFET's by as much as a factor of 30, at 77K. Nevertheless, researchers have not considered the advantages of cryogenically cooled components operating together in a circuit configuration, for example, because of the refrigeration cost associated with cryogenically cooled electronics. Cryocooled electronics have been considered for enhanced noise performance of amplifiers, and to speed up computer circuitry.
As will be described in more detail below, one particularly advantageous use of cryogenically cooled electronics is in high power switching power supplies. To understand those advantages more detail, however, a brief discussion of the limitations of such power supplies operating at room temperature needs to be considered. Switch mode amplifiers, often called class D amplifiers, regulated power supplies, and frequency converters became a reality with the introduction of high speed power silicon devices. The main advantages of these switch mode applications is that, at least for ideal devices, the only losses involved are the saturation losses of the power devices in the forward direction. These losses are very low compared to the losses sustained in linear regulation or amplification devices; and these low losses have allowed the physical size of regulated power supplies to be reduced considerably.
For example, a linearly-regulated, three voltage, 20.degree. watt power supply was typically over a cubic foot in volume, contained a heavy and expensive power transformer with its associated filtering components, and required approximately 200 watts of dissipation capacity in the active devices. In the newer switch mode versions of these 200 watt power supplies, which appear in nearly every personal computer, the volume of the supply has been reduced between four and six times, with comparable reductions in weight, power requirements, and cost.
In accordance with the prior art, the typical method used to obtain these size reductions has been to replace conventional power frequency components with significantly smaller filter and active components which operate at 300 to 30,000 times the frequency of the older power supplies. The power-speed product of the active devices, and the thermal limitations of the filter components, has slowed progress in the design of these switching mode devices. The most common power supply sizes range from a few watts (20-800 kHz) through a few kilowatts (18-35 kHz). While larger amplifiers and supplies have been built (5-50 KW (5-20 kHz)), they are rare.
There are very few switching mode power supply designs which operate above these speed-power levels, unless the design is based on integrating multiple small modules, or using multiple active devices at lower frequencies with multiple passive components of smaller rating to limit the thermal problems. In general, the losses scale at least linearly with the frequency of operation. Therefore, very high power density designs, operating at high frequency, are limited to a low power output. These losses are often dominated by the switching losses in the power devices.
Referring to FIGS. 1 and 2, in a typical prior art switch mode power supply design, (and a simplest design), a so-called non-isolated Buck converter has a DC input voltage V+, a DC output voltage V.sub.out, an active switching device illustrated as an NPN transistor, Q.sub.1, a rectifying diode D.sub.1, a series inductor L.sub.1, a capacitor C.sub.1, and a resistive load R. The output of this switching power supply is described by the equation: EQU V.sub.out =(t/T)V.sub.+ =kV.sub.+ (Equation 1)
where "k" is the duty cycle. The voltage and current across the inductor L.sub.1 is illustrated in FIG. 2.
Referring to FIG. 3, in other architectures for amplifiers, power supplies and frequency convertors, the next level of complexity comes by adding a second inductor L.sub.2, and isolating the input from the output using a capacitor. This is often done when an output voltage polarity different from an input voltage polarity is required. Referring to FIG. 3, the addition of the second reactive element L.sub.2 makes it possible, by isolating the input and output, to provide an output voltage having a different polarity than the input. Here, the active device is Q.sub.1 is operated in a shunt mode. The operation of the power supply of FIG. 3 is more complex than the power supply of FIG. 1, and is detailed in Hnatnek, "Design of Solid State Power Supplies", Third Edition, 1989, Van Nostrand Rheinhold, at pages 160 and following. The DC isolation is provided by capacitor C.sub.1 and the drive to the base of the active device Q.sub.1 turns the transistor hard on for a time t with a period T. When Q.sub.1 turns on, a voltage V.sub.+ is developed across L.sub.2 and the current in L.sub.2 begins to rise at a rate of V.sub.+ /L.sub.2. When Q.sub.1 turns off, the current L.sub.2 continues to flow into C.sub.1 through the rectifier D.sub.1. Eventually capacitor C.sub.1 charges to a constant voltage V.sub.c. The equations which describe the on and off operation of the circuit are given in two pairs below. Equations 2 and 3 describe the operation when Q.sub.1 is on and the equations 4 and 5 describe the operation when Q.sub.1 is off. EQU V.sub.+ =L.sub.2 dI.sub.1 /dt (Equation 2) EQU V.sub.c =L.sub.1 dI.sub.2 /dt+I.sub.2 R.sub.L (Equation 3) EQU V.sub.+ -V.sub.c =L.sub.2 dI.sub.1 /dt (Equation 4) EQU O=L.sub.1 dI.sub.2 /dt+I.sub.2 R.sub.L (Equation 5)
In order to obtain reasonably ripple free output from the circuit of FIG. 3, and the potential for negative output voltage, the values of the inductors L.sub.1 and L.sub.2 must be large. Thus, the circuit of FIG. 3 requires large inductors and capacitors for proper operation of the circuit where high power is to be provided. When the power supplies of FIGS. 1 and 3 are operated in a low power environment, the component values are typically kept small, the loss is tolerated, and the frequency of operation (1/T) is raised to as high a value as possible. In high power circuits, this is not usually an option without substantially increasing the component count and developing strategies for removing the heat from both the capacitors and the inductors.
Thus, as the power output of the switch mode power supply increases, one runs into severe "component constraints" as are detailed in Hnatnek, referred to above. The components of a power supply such as those described in FIGS. 1 and 3, are subject to very high electrical stress; and in particular, the components subject to the most stress are the switching transistor, the diode, and the coupling or filter capacitors.
It is therefore an object of the invention to provide a power supply circuit which can produce substantial power, on the order of greater than 100 watts and as much as, for example, 50 kilowatts or more, which is reliable, which has a substantially reduced size, which reduces stress on the components being employed, and which delivers substantially improved performance using high temperature superconductor inductors. A further object of the invention is a power supply, especially in larger sizes, which is considerably less expensive than a supply based on conventional technology.